In the manufacture of printed circuit cards and boards, a dielectric sheet material is employed as the substrate. A conductive circuit pattern is provided on one or both of the major surfaces of the substrate.
A conductive pattern can be formed on the surface of the substrate using a variety of known techniques. These known techniques include the subtractive technique and the additive technique. In the subtractive technique, a blanket layer of copper on the substrate is selectively etched to form the desired circuit pattern. It is well known to etch a pattern in the metallic surface such as copper by applying to the surface a photoresist, exposing the photoresist through a pattern of actinic radiation, removing the exposed or unexposed portion of the resist depending upon the type of resist to expose the underlying metal, and then etching the exposed metal with a suitable etchant. In the additive process, typically the photoresist is applied to the surface of the substrate, followed by being exposed through a pattern to actinic radiation and removing the exposed or unexposed portions of the resist depending upon the type of resist employed to expose the underlying substrate and the desired circuitry to be subsequently provided. Next, the exposed substrate is typically coated with a metallic layer such as copper from an electroless plating bath followed by electroplating to achieve the desired line thickness.
However, the bond of the photoresist or photoactive layer to the metal surface has not always been adequate, especially for exposure to various plating baths and/or etching compositions. However, as can be appreciated, the adhesion of photoresists to the underlying metallic surface is critical not only in fine-line subtractive circuitization, but also in other photoresist-based manufacturing processes such as pattern electroplating and solder mask applications. In solder mask applications, a photoactive polymeric solder mask is applied and defined by photolithographic techniques to uncover those underlying portions whereby solder is to be deposited while protecting other areas from having the solder deposit.
In view of the adhesion problems between these polymers and underlying metallic surfaces, a number of surface texturing/screening processes have been suggested for enhancing such adhesion. Some examples include providing pumice on foil copper to roughen its surface in the case of subtractive circuitization, sulfuric acid pretreatment of additive copper surfaces in the case of Sn/Pb electroplating, copper oxide treatment of copper in the case of certain solder mask applications, and vapor blast treatment of gold in the case of certain solder mask applications to chip carriers.
However, despite the attempts to enhance adhesion, failures which in turn result in the scrapping of product still occurs periodically. The adhesion existing between photoactive polymers and underlying metallic substrates remains borderline, at most, in many cases. For instance, it has been observed that the interface between additive copper and various photoresists is susceptible to separation during immersion tin and subsequent Sn/Pb electroplating due to increased immersion times, photoresist mis-registration, and rinse impurities that might be present in the sulfuric acid pre-clean prior to applying the resist.
Accordingly, the need remains for providing enhanced adhesion between photoactive polymers and metallic surfaces in order to yield more robust as well as potentially more cost-effective, photoactive-based processes.